1. Technical Field
The present invention relates to what are known as liquid developer-type image forming apparatuses and image forming methods that form an image by developing a latent image using a liquid developing agent having toner and a carrier liquid and transferring that image onto an image bearing belt.
2. Related Art
With this type of image forming apparatus, an endless intermediate transfer belt (this corresponds to an “image bearing belt” according to the invention) that moves cyclically in a predetermined direction is provided, and yellow, magenta, cyan, and black image formation units are provided along the intermediate transfer belt corresponding to each of four mutually different primary transfer locations, as described in, for example, JP-A-2009-157327 (FIG. 1). At each image formation unit, a latent image is formed upon an image bearing member, and the latent image is developed using a liquid developing agent containing a carrier and toner particles, thus forming an image. A color image is then formed by superimposing these images upon the intermediate transfer belt. Accordingly, with this type of image forming apparatus, it is extremely important, from the standpoint of improving the image quality, for each single-color image to be superimposed according to a predetermined positional relationship. In other words, suppressing registration shift is necessary in order to achieve superior image quality.
Here, temperature changes within the apparatus act as a cause of the occurrence of registration shift. In other words, there are situations where, if the temperature within the apparatus changes, the intermediate transfer belt expands/contracts, or roller diameters change and cause fluctuations in belt speeds, in accordance with the temperature change. As a result, there have been situations where the images of each color transferred onto the intermediate transfer belt have shifted relative to each other, which in turn leads to a drop in the image quality.
Accordingly, applying a technique such as that disclosed in, for example, JP-A-8-101554, can be considered. That is, with the apparatus disclosed in JP-A-8-101554 (FIG. 9), two sensors are arranged in the movement direction of an endless belt, and the times at which an opening hole provided in the endless belt passes the two sensors are detected thereby. The belt speed is also found based on the result of that detection, and the formation timing of the latent image is controlled in correspondence therewith. Accordingly, when this technical spirit is applied to the so-called liquid developer-type image forming apparatus disclosed in JP-A-2009-157327, the relative positions of the latent images are adjusted even if the belt speed fluctuates, thus correcting registration shift.
Incidentally, a liquid developing agent in which toner particles are dispersed throughout a carrier liquid is used in such liquid developer-type image forming apparatuses, and therefore, as has been generally known thus far, there is a trend for the transfer efficiency thereof to be less than so-called dry-type image forming apparatuses. Accordingly, forming winding portions by winding the intermediate transfer belt upon each of the image bearing members and performing the primary transfer of the images formed upon the image bearing members onto the intermediate transfer belt at those winding portions has been proposed.
However, at each winding portion, the region at which the image bearing member and the intermediate transfer belt make contact with each other is broad, and thus the time variation of the belt resistance value is greater than that with dry-type image forming apparatuses. In other words, the longer the operating time of the apparatus becomes, the greater the variation in the belt resistance values grows. Furthermore, if the intermediate transfer belt is replaced, there is a wide discrepancy between the belt resistance values prior to and following the replacement. There are thus situations in which the voltages at the primary transfer locations fluctuate greatly. In addition, because this region is broad, there have been situations in which the voltages at the primary transfer locations have been affected even with different image patterns.
With such a liquid developer-type image forming apparatus that has winding portions, fluctuations in the voltages occur with ease at the primary transfer locations, and as will be described later with reference to FIG. 5, friction arising between the image bearing members and the intermediate transfer belt at the primary transfer locations changes due to the stated fluctuations in the voltages. As a result, even if the belt speed is accurately measured using the technique disclosed in JP-A-8-101554, the belt speed will fluctuate due to the fluctuation in the friction, and thus is it difficult to carry out accurate registration adjustment.